llvm_*,clang_*: allow src overrides

pkgs/development/compilers/llvm/6/clang/default.nix

@@ -8,8 +8,10 @@ let
   self = stdenv.mkDerivation ({
     name = "clang-${version}";
 
+    src = fetch "cfe" "0rxn4rh7rrnsqbdgp4gzc8ishbkryhpl1kd3mpnxzpxxhla3y93w";
+
     unpackPhase = ''
-      unpackFile ${fetch "cfe" "0rxn4rh7rrnsqbdgp4gzc8ishbkryhpl1kd3mpnxzpxxhla3y93w"}
+      unpackFile $src
       mv cfe-${version}* clang
       sourceRoot=$PWD/clang
       unpackFile ${clang-tools-extra_src}

pkgs/development/compilers/llvm/6/llvm.nix

@@ -19,8 +19,6 @@
 let
   inherit (stdenv.lib) optional optionals optionalString;
 
-  src = fetch "llvm" "1qpls3vk85lydi5b4axl0809fv932qgsqgdgrk098567z4jc7mmn";
-
   # Used when creating a versioned symlinks of libLLVM.dylib
   versionSuffixes = with stdenv.lib;
     let parts = splitString "." release_version; in
@@ -30,8 +28,10 @@ in
 stdenv.mkDerivation (rec {
   name = "llvm-${version}";
 
+  src = fetch "llvm" "1qpls3vk85lydi5b4axl0809fv932qgsqgdgrk098567z4jc7mmn";
+
   unpackPhase = ''
-    unpackFile ${src}
+    unpackFile $src
     mv llvm-${version}* llvm
     sourceRoot=$PWD/llvm
   '';
@@ -146,8 +146,6 @@ stdenv.mkDerivation (rec {
 
   enableParallelBuilding = true;
 
-  passthru.src = src;
-
   meta = {
     description = "Collection of modular and reusable compiler and toolchain technologies";
     homepage    = http://llvm.org/;